Biogas, principally methane and carbon dioxide, may be produced from feedstocks comprising organic matter, either as a slurry or sludge, for example, sludge from wastewater treatment, treatment of sludge produced in water purification, or treatment of particulate biodegradable organic waste. Processes to produce biogas typically involve the treatment of complex organic material, both the liquid and residual solids fractions, using thermal hydrolysis. Techniques that employ both thermal hydrolysis and anaerobic digestion are directed to the treatment of a limited group of organic material feedstocks, for example wastewater treatment bio-solids.
The treatment of wastewater treatment of bio-solids by thermal hydrolysis and anaerobic digestion produces a carbonaceous material that is reduced in volume compared to the feedstock and may be landfilled, used as a soil additive, or as a fuel to generate energy. However, these processes treat the liquid fraction produced following thermal hydrolysis, are not configured to maximize the manufacture of biogas using anaerobic digestion.
U.S. Pat. No. 7,311,834 teaches an apparatus for the treatment of particulate biodegradable organic waste using thermal hydrolysis and anaerobic treatment. The process uses thermal hydrolysis at a temperature from 130 to 225° C., and at a pressure at or above the saturated water vapor pressure, to produce a solubilized organic material liquid fraction and residual solids. The solubilized organic material is separated from the residual solids, and the solubilized organic material is treated by anaerobic digestion to produce methane rich biogas.
U.S. Pat. No. 7,101,482 discloses a process for treating sludge obtained from biological treatment of effluents for example town or industrial wastewater. The process involves solubilization of the sludge using thermal hydrolysis (between 50° C. and 180° C., and at a pressure between 2 to 40 bars), separation of the liquid and solid fractions using a liquid/solid separator. The separated solids fraction is recycled for further solubilization, while the liquid fraction is treated directly in a biological treatment process for the manufacture of biogas.
U.S. Pat. No. 6,966,989 teaches the use of high pressure, high temperature thermal hydrolysis for treating sludge containing fermentable organic material, for example obtained from urban or industrial wastewaters, or sludges originating from sewage main cleaning operations. The products from thermal hydrolysis (130 to 200° C., and a pressure of between 10 Bar to 20 Bar) may be combined with a mesophilic or thermophilic anaerobic digester for the production of biogas.
U.S. Pat. No. 7,332,095 teaches a method of wet oxidation of organic material obtained from industrial or consumer waste, at an amount of less than 10% (by weight) within the feedstock. The process involves thermal treatment (100 to 300° C. and a pressure between 1.4 to 13 MPa) of the dilute feedstock in presence of a catalyst. The vapour phase is condensed to obtain acetic acid, formic acid and CO2, and the liquid phase is recycled for further thermal treatment.